Nested and isolated transistors with reduced impedance difference

ABSTRACT

A processing layer, such as silicon, is formed on a metal silicide contact followed by a metal layer. The silicon and metal layers are annealed to increase the thickness of the metal silicide contact. By selectively increasing the thickness of silicide contacts, R s  of transistors in iso and nested regions can be matched.

FIELD OF THE INVENTION

The present invention relates to integrated circuits (ICs). Moreparticularly, the invention relates to reducing the difference inresistance between nested and isolated transistors.

DESCRIPTION OF THE RELATED ART

The fabrication of ICs involves the formation of features on a substratethat make up circuit components, such as transistors, resistors andcapacitors. The circuit components are interconnected as desired toachieve the desired function.

FIG. 1 shows a conventional transistor 110. The transistor is formed ona doped well 108. The transistor has source/drain diffusion regions 125a-b adjacent to a gate 130, which includes a gate electrode 131 overgate dielectric layer 132. The gate electrode typically is formed frompolysilicon. Located on the gate sidewalls are dielectric sidewallspacers 160. Metal silicide contacts 140 and 141 are provided on thesurface of the substrate in the source/drain regions and gateelectrodes. The distance L_(G) between the source/drain regions underthe gate is referred to as the gate length. Isolation regions 180 can beprovided within substrate 105.

ICs can include p-type, n-type or both types of transistors. Typically,the IC includes areas which have densely populated transistors (nestedregions) or less densely populated transistors (iso regions).Conventional processes for forming ICs result in transistors havingdifferent operating parameters, such as switching speed. This isundesirable as this requires IC designs to accommodate the transistorswith the slowest switching speed, impacting overall IC performance.

From the foregoing, it is desirable to provide transistors in ICs havingsimilar operating parameters.

SUMMARY OF THE INVENTION

The present invention relates to a method of forming a contact or an IC.The method includes providing a substrate with a metal silicide contact.A processing layer, such as silicon, is formed on the metal silicidecontact. The processing layer can be formed by CVD, ALD or ionimplantation. A metal layer is deposited over the processing layer. Thesubstrate is processed to cause a reaction between the processing andmetal layers, thereby increasing the thickness of the metal silicidecontact. The steps of forming a processing layer, depositing a metallayer and processing the substrate can be repeated until the thicknessof the contact is at a target thickness.

In another aspect, the invention relates to an IC. The IC includes asubstrate on which the IC is formed. The substrate has first and seconddevice regions defined thereon. The defined regions are populated withsemiconductor devices having contacts, such as transistors. The firstregion has more densely populated devices than the second device region.The R_(s) of the devices in the device regions are substantially thesame.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become apparent throughreference to the following description and the accompanying drawings.Furthermore, it is to be understood that the features of the variousembodiments described herein are not mutually exclusive and can exist invarious combinations and permutations.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. Various embodiments of thepresent invention are described with reference to the followingdrawings, in which:

FIG. 1 shows a conventional transistor;

FIGS. 2 a-b show simulated series resistance of p-type and n-type isoand nested transistors respectively as a function of gate length;

FIG. 3 shows a portion of an IC in accordance with one embodiment of theinvention;

FIG. 4 shows a process of forming metal silicide contacts in accordancewith one embodiment of the invention; and

FIGS. 5 a-f show a process for forming an IC in accordance with oneembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to improving performance parameters oftransistors, such as switching speed. In one embodiment, the inventionrelates to providing transistors having consistent performanceparameters on the IC. For example, nFETs or ICs in both iso and nestedregions have consistent performance parameters.

We have discovered that series resistance (R_(s)) of gate electrodes aredifferent in the nested and iso regions. FIGS. 2 a-b show simulatedR_(s) of p-type transistors and n-type transistors as a function of gatelength (L_(G)) in nested and iso regions. R_(s) of transistors weresimulated at L_(G) of 48 μm, 56 μm, 64 μm, 80 μm and 110 μm and measuredat 25° C. R_(s) is determined by the following equations:

R _(s)=50 mV/(I _(dlin) @VG−V _(Tlin))=−1.1V for p-type transistors; and

R _(s)=50 mV/(I _(dlin) @VG−V _(Tlin))=1.1V for n-type transistors,

where

I_(dlin) is the drain current measured at the linear region and

V_(Tlin) is the threshold voltage measured at the linear region.

For iso transistors, the pitch is 0.384 um while nested transistors havea pitch of 0.182 um. From the figures, R_(s) for nested transistors(solid line) is higher than that of iso transistors (broken line). Forp-type transistors, the R_(s) converge as L_(G) approaches zero.However, for n-type transistors, R_(s) remains different as L_(G) isequal to zero, for example, a delta of about 10%. Due to the R_(s)delta, transistors in nested and iso regions will perform differently,such as switching speed. The transistors with higher R_(s) will switchslower than the transistors with lower R_(s). This results in overallperformance degradation of the IC since the slower switching transistorsmust be accommodated.

FIG. 3 shows a portion 300 of an IC in accordance with one embodiment ofthe invention. The IC, for example, can be any type of IC, such asdynamic random access memories (DRAMs), static random access memories(SRAMs), non-volatile memories including programmable read-only memories(PROM) and flash memories, optoelectronic devices, logic devices,communication devices, digital signal processors (DSPs),microcontrollers, system-on-chip, as well as other types of devices.Other types of ICs are also useful.

The portion of the IC comprises a substrate 305. The substrate, forexample, comprises a semiconductor substrate, such as silicon. Othertypes of substrates, such as silicon on insulator, are also useful. Thesubstrate, in one embodiment, comprises a lightly doped p-typesubstrate. As shown, the substrate includes at least first and seconddevice regions 392 and 394. The first and second regions correspond to,for example, nested and iso regions. The nested and iso regions aredefined by first and second doped wells 308 and 388. The doped wellscomprise dopants having a first polarity type. The wells serve as activeregions of the substrate, separated by isolation regions 380. Theisolation regions comprise, in one embodiment, shallow trench isolations(STIs). Other types of isolation regions can also be used.

A plurality of transistors 310 and 360 are provided in the nested andiso regions. A transistor includes a gate stack 330 and source/draindiffusion regions 325. The gate stack generally comprises polysilicongate electrode 331 over a gate dielectric 332. Dielectric liner andspacers 360 can be provided. The liner lines the gate and substratewhile the spacers are disposed on the gate sidewalls. Typically theliner comprises oxide while the spacers are formed from nitride. Othercombinations of materials are also useful. Nested transistors areprovided with metal silicide contacts 340 and 341 over the diffusionregions and gate electrode; iso transistors are provided with metalsilicide contacts 381 and 382 over the diffusion regions and gateelectrode. The metal silicide contacts, for example, comprise nickel orits alloy, such as NiPtSi. Other types of metal, for exampleCo_(x)Si_(y), are also useful. The contacts serve as terminals of thetransistors.

Transistors in the nested region are densely populated. For example, thegates of the transistors have a pitch which is equal to about groundrule. Typically, adjacent transistors in the nested region sharediffusion regions (e.g., 310 a-b). Other transistor configurations inthe nested region are also useful. In contrast, transistors in the isoregion are less densely packed. The gates of the iso transistors have apitch which is greater than that of the nested transistors, for example,at least about 2 times. The transistors may be individual transistorswhich do not share diffusion regions. As shown, the portion of the ICincludes three nested transistors and one iso transistor. It isunderstood that actual ICs have many more nested and iso transistors aswell as other types of components.

In one embodiment, the first polarity type comprises p-type dopants,forming p-wells for n-type (second polarity type) transistors. Thetransistors in the iso and nested regions, in one embodiment, comprisesabout the same R_(s). For example, the difference in R_(s) between thetwo regions should be less than 5%. In one embodiment, metal silicidecontacts of nested and iso transistors comprise about the same thicknessto produce about the same R_(s).

FIG. 4 shows a process 470 for reducing R_(s) in contacts of, forexample, transistors. In one embodiment, the process includes formingmetal silicide contacts having a thickness to achieve the desired R_(s).The process commences by providing a substrate prepared for formingmetal silicide contacts at step 471. The substrate, for example,includes contact regions, such as doped diffusion regions and gateelectrodes of transistors. The surface of the substrate is prepared by apresalicide clean, such as SP+SC1+SC2. A metal layer is deposited on theprepared substrate at step 472. The metal layer, for example, comprisesnickel or alloys thereof, such as NiPtSi. Other types of metals oralloys, such as Co_(x)Si_(y), are also useful. The metal layer can bedeposited by various techniques, such as sputtering. Other techniquesare also useful.

At step 473, the substrate is processed. In one embodiment, a salicideprocess is performed. The salicide process comprises, for example,annealing the substrate. Typically, the substrate is annealed at 500° C.spike in N₂-O₂. The salicide process causes a reaction between thesilicon and metal to form metal silicide in the contact region. Excessmetal such as unreacted metal over the non-contact regions as well as inthe contact regions are removed at step 474. Typically, a HCl/HNO₃ etchis performed to remove excess metal. The thickness of the metal silicidecontact can be, for example, about 180-220 Å.

Depending on whether the metal silicide contact has a thickness toproduce the desired R_(s) at step 475, the process proceeds to eitherstep 476 or 477. If the metal silicide contact needs to be thicker, theprocess continues to step 477. At step 477, a processing layer is formedon the substrate, covering the transistors. The processing layer, in oneembodiment, comprises silicon. The processing layer is formed by, forexample, CVD. The processing layer can be polycrystalline or amorphoussilicon. The thickness of the processing layer is about 50 Å. Otherthicknesses are also useful. Alternatively, the processing layer can beformed by implanting silicon ions on the substrate. Excess silicon onthe substrate, for example, in regions other than the contact regions isremoved. Removal can be achieved with reactive ion etching (RIE). Steps472-475 are repeated until the metal silicide contacts have a thicknessto produce the desired R_(s). After the metal silicide has the desiredthickness, the process continues at 476 to form the IC.

FIGS. 5 a-f show a process for forming an IC in accordance to oneembodiment of the invention, the IC comprising a first and a seconddevice regions having transistors. In one embodiment, the first deviceregion is a nested region wherein transistors are located in closeproximity relative to each other. The pitch between the transistors inthe nested region, for example, is about 180 nm. The second deviceregion is an iso region wherein transistors are located further apart asopposed to transistors in the nested area. The pitch between thetransistors in the iso region, for example, is about 384 nm.

Referring to FIG. 5 a, an IC 500 comprising a first device region 592and a second device region 594 is illustrated. In one embodiment, thefirst device region is a nested region and the second device region isan iso region. Transistors 510 are formed within the first and seconddevice region. A semiconductor substrate 505 is provided. The substrate,in one embodiment, comprises a silicon substrate. Other types ofsubstrates, such as a lightly p-doped silicon or SOI, may also beuseful. The substrate is prepared with active regions 508 and 588. Theactive regions each comprise a heavily doped region with dopants of afirst polarity type. To form the active regions, conventional ionimplantation techniques, such as implantation with a mask can be used.Other techniques for forming the active region are also useful.

Isolating the active regions from other device regions on the substrateare isolation regions 580. The isolation regions, for example, compriseSTI regions. Various conventional processes can be employed to form theSTI regions. For example, the substrate can be etched using conventionaletch and mask techniques to form trenches which are then filled withdielectric material such as silicon oxide. Chemical mechanical polishing(CMP) can be performed to remove excess oxide and provide a planarsubstrate top surface. The STI regions can be formed, for example, priorto or after the formation of the doped wells.

Gate layers are formed on the substrate. Forming the gate layers, forexample, comprises sequentially forming a gate dielectric layer 532 anda gate electrode layer 531 on the substrate surface. In one embodiment,the gate dielectric layer comprises thermal oxide. Other types of gatedielectrics are also useful. The gate electrode layer, for example,comprises polysilicon. Other types of gate electrode materials, such asmetal gate, are also useful. The polysilicon can be in-situ doped ordoped by ion implantation after deposition. Providing undopedpolysilicon is also useful.

Spacers 560 are formed on the gate sidewalls. The spacers comprise, forexample, a dielectric material such as oxide and/or nitride. The spacerscan be formed using conventional spacer processes, such as depositing ablanket dielectric layer on the substrate and gate. The dielectric layeris then patterned to remove the horizontal portions, leaving the spacersat the gate sidewalls.

Source/drain diffusion regions 525 a-b are formed in the active regionof the substrate adjacent to the gate. The diffusion regions, forexample, comprise heavily doped regions of the first polarity type. Thedoped regions can be formed by ion implantation. The implant can beself-aligned or formed using an implant mask. Other techniques forforming the diffusion regions are also useful. In one embodiment, thediffusion regions comprise lightly doped and heavily doped regions via,for example, a two step ion implantation process. For example, thelightly doped regions are formed after the gate is patterned and theheavily doped regions are formed after spacer formation. After theformation of diffusion regions, the dopants are activated by, forexample, a thermal annealing process. The annealing also serves tofacilitate recovery from any deformation to the crystal structureincurred during the process of ion implantation. Typically, the annealis carried out at abut 1080° C. with a 220° C./minute ramp up rate. Inother embodiments, the activation of dopants can be integrated intosubsequent processing steps.

In one embodiment, referring to FIG. 5 b, a pre-silicide clean can beperformed. The pre-silicide clean, for example, comprises wet cleaningusing DHF in a dilute aqueous solution. Other clean chemistries orprocesses are also useful. The pre-silicide clean prepares the substratefor metal deposition by removing native oxide on the gate andsource/drain diffusion regions. The clean process, for example, formsundercuts 564 in the bottom of the spacers.

A metal layer 574 is deposited on the substrate, covering thetransistors. In one embodiment, the metal layer comprises nickel ornickel-alloy such as NiPtSi. Other types of metals, for example,Co_(x)Si_(y), are also useful. The thickness of the metal layer, in oneembodiment, is about 220 Å.

Referring to FIG. 5 c, a salicide process is preformed to form metalsilicide contacts 540 and 541 over the diffusion regions (525 a-b) andthe gate electrode (531). In one embodiment, the salicide process formsnickel or nickel alloy silicide contacts. Other types of metal-silicidecontacts are also useful. The salicide process, for example, annealingthe substrate, causing a reaction between the metal and silicon. In oneembodiment, the substrate is spiked annealed at a temperature of about500° C. in N₂-O₂ ambient. Other process parameters may also be useful.The excess or unreacted metal on the substrate, such as over the STIsand gate sidewalls, is removed. Removal can be achieved using aselective wet etch clean to the metal material, such as nickel.

As discussed, the R_(s) of nested transistors are higher than the R_(s)of iso transistors. To compensate for the difference of R_(s) of thedifferent types of transistors, the thickness of the metal silicidecontacts of the nested transistors are increased. In one embodiment, themetal silicide contacts of the nested transistors are further processed.

As shown in FIG. 5 d a mask layer 555 is deposited over the isolatedarea to protect transistors therein from processing. A processing layer550 is formed on the substrate, covering the mask and transistors in thenested region. The processing layer, in one embodiment, comprisessilicon. The silicon, for example, can be polysilicon or amorphoussilicon formed by CVD. Other techniques, such as ALD is also useful. Thethickness of the processing layer, for example, is about 50 Å. In analternative embodiment, the processing layer is formed by implantingsilicon onto the substrate.

The excess processing layer on the substrate, such as over the STIs andgate sidewalls, are removed as shown in FIG. 5 e. In one embodiment, theexcess processing layer is removed by selective RIE.

Referring to FIG. 5 f, a metal layer 545 is deposited over thesubstrate, covering the diffusion regions and gate. Preferably, themetal layer comprises nickel or nickel alloy, such as NiPt or NiPtSi.Typical thickness of the metal layer is about 220 Å. A silicidationprocess is performed. The silicidation process comprises, for example,annealing the substrate to increase the thickness of the metal silicidecontacts. Excess or unreacted metal is removed. The processes of forminga processing layer, stripping unwanted processing layer from thesubstrate, depositing a metal layer, silicidation process, and removingexcess metal are repeated until the desired R_(s) is achieved. Theprocess of forming the IC continues. For example, processes of formingadditional device components, interconnects, passivation and package areperformed.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The foregoingembodiments, therefore, are to be considered in all respectsillustrative rather than limiting the invention described herein. Scopeof the invention is thus indicated by the appended claims, rather thanby the foregoing description, and all changes that come within themeaning and range of equivalency of the claims are intended to beembraced therein.

1. A method of forming an IC comprising: providing a substrate with ametal silicide contact; forming a processing layer on the metal silicidecontact; depositing a metal layer over the processing layer; and furtherprocessing the substrate to increase a thickness of the metal silicidecontact.
 2. The method of claim 1 wherein: providing a substratecomprises first and second device regions defined thereon, the first andsecond device regions being populated with transistors with contactregions having silicide contacts.
 3. The method of claim 2 whereinforming the processing layer comprises forming a silicon layer on themetal silicide contact.
 4. The method of claim 2 wherein the transistorscomprises n-type transistors.
 5. The method of claim 4 wherein formingthe processing layer comprises forming a silicon layer on the metalsilicide contact.
 6. The method of claim 2 wherein: the first deviceregion comprises nested transistors and the second device regioncomprises iso transistors; and forming the processing layer, depositingthe metal layer and further processing the substrate are performed onthe nested transistors in the first region.
 7. The method of claim 6wherein the transistors comprises n-type transistors.
 8. The method ofclaim 6 wherein forming the processing layer comprises forming a siliconlayer on the metal silicide contact.
 9. The method of claim 8 whereinthe transistors comprises n-type transistors.
 10. The method of claim 2wherein forming the processing layer, depositing the metal layer andfurther processing the substrate are repeated until the thickness of themetal silicide contacts is equal to a target thickness.
 11. The methodof claim 10 wherein the target thickness results in R_(s) of the nestedtransistors and R_(s) of the iso transistors being substantially thesame.
 12. The method of claim 10 wherein the target thickness results inR_(s) of the nested transistors and R_(s) of the iso transistors havinga difference of less than or equal to 5%.
 13. The method of claim 1wherein forming the processing layer comprises forming a silicon layeron the metal silicide contact.
 14. The method of claim 10 whereinforming the processing layer on the metal silicide contact comprisesCVD, ALD or ion implantation.
 15. The method of claim 1 wherein themetal silicide contact comprises a contact of a n-type transistor. 16.The method of claim 1 wherein forming the processing layer, depositingthe metal layer and further processing the substrate are repeated untilthe thickness of the metal silicide contact is equal to a targetthickness.
 17. The method of claim 16 wherein the target thicknessresults in a target R_(s).
 18. A method of forming contacts comprising:providing a substrate with a metal silicide contact; forming aprocessing layer on the metal silicide contact; depositing a metal layerover the processing layer; and further processing the substrate toincrease a thickness of the metal silicide contact.
 19. An integratedcircuit (IC) comprising: a substrate on which the IC is formed; firstand second device regions defined on the substrate, the first and seconddevice regions being populated with semiconductor devices havingcontacts; wherein the first device region is more densely populated withsemiconductor devices than the second device region and the R_(s) of thefirst device region and the second device region are substantially thesame.
 20. The IC of claim 19 wherein a difference of R_(s) in the firstdevice region and the second device region is less than or equal to 5%21. The IC of claim 19 wherein the transistors comprises n-typetransistors.